The prompt phase of GRBs

1. The prompt phase of GRBs Dimitrios Giannios Lyman Spitzer, Jr. Fellow Princeton, Department of Astrophysical Sciences Raleigh, 3/7/2011

2. Structure of the talk • Main properties of the prompt emission • Models for the GRB flow • Fireballs • Internal shocks • Poynting-flux dominated flows • Magnetic Reconnection • Radiation region • Thomson thin vs photospheric emission for the GRB • Fermi LAT bursts • Correlations: what can we learn for the central engine?

3. Gamma-ray bursts: spectra and variability Nph(t) νfν E (MeV) t (sec)

4. GRBs: ultrarelativistic jets • Clues • The prompt emission has • non-thermal spectral appearanceBand et al. 1993; Preece et al. 1998 • Rapidvariability • The GRB-emitting flow is ultrarelativistic(γ>100, 300, 1000?)e.g. Piran 1999… • Big questions • Type of central-engine/Jet composition • How is the flow accelerated? • Which processes result in the observed GRB emission?

5. Central Engine My focus: why and how do jets radiate? ? Acceleration Internal dissipation External interactions

6. (How to tell a millisecond magnetar) • A millisecond neutron star has rotational energy • Extracted on a timescale of ~30 sec for Usov 1992; Thompson 1994; Uzdensky & MacFadyen 2006; Metzger et al. 2007; 2011 • After the GRB we are left with a supermagnetar! • Contains • The magnetic field decays fast (100-1000yr; Thompson & Duncan 1996) • May power SGR superflares ~100 times more powerful than that of SGR 1806-20 in December 2004! DG 2010

7. The driving mechanism: MHD Energy Extraction and/or neutrino annihilation B-fields extract rotational energy from the compact object/inner accretion disk at a rate Neutrino annihilation energy deposition rate (erg cm –3 s-1) Blandford & Znajek 1977 Koide et al. 2001 van Putten 2001 Lee et al. 2001 Barkov & Komissarov 2008 Usov 1992 Uzdensky & McFadyen 2007 Bucciantini et al. 2007 Metzger et al. 2010 Ruffert & Janka 1999; Popham et al. 1999; Aloy et al. 2000; Chen & Beloborodov 2007; Zalamea & Beloborodov 2011

8. General considerations: Acceleration • Important quantities of the flow: • luminosity L • mass flux • Efficient acceleration can lead to γsr~η • Depending on the energy extraction mechanism, the flow can be dominated by • Thermal energy  thermal acceleration (Fireball) Paczynski 1986; Goodman 1986; Sari & Piran 1991 • Magnetic energy  MHD acceleration (Poynting-flux dominated flow) Usov 1992; Thompson 1994; Mészáros & Rees 1997; Drenkhahn & Spruit 2002; Lyutikov & Blandford 2003 Baryon loading

9. Fireballs • Parameters: L,η,initial radiusro • Go through fast acceleration • Converting thermal energy into kinetic • Saturation takes place when • almost all thermal energy is used: γsr η • at the photospheric crossing γsr < η • Radiation and matter decouple when τ~ 1 • Photospheric emission takes place internal shocks photospheric emission thermal component energy content kinetic component τ~1 distance r

10. Strongly magnetized jets • Recent progress in 2D axisymmetric relativistic MHD simulations & theory Vlahakis & Koenigl 2003; Komissarov et al. 2009; 2010; Tchekhovskoy et al. 2009; 2010; Lyubarsky 2009; 2010 • High magnetization flows accelerate to Γ>>1, Butmost of the energy remains in the B field • Shocks are inefficient • Dissipative MHD processes are key to jet emission (and acceleration) • Non-axisymmetric instabilities may develop a large distance leading to dissipation and emission e.g., Lyutikov & Blandford 2003; Narayan & Kumar 2008; Zhang & Yan 2011

11. The reconnection model for GRBs • The field is in general not axisymmetric at the central engine thermal photospheric emission • Model for GRBs: Magnetic field changes polarity on small scales and reconnects vrec=εcDrenkhahn 2002 and Denkhahn & Spruit 2002; see also McKinney & Uzdensky 2011 • Dissipation is gradual and leads to acceleration of the flow and heating of plasma • The model predicts a strong photospheric component and optically thin dissipation energy content thin emission magnetic component kinetic component ⨀ ⨀ ⨀ ⨀ ⨀ ⨀ ⨀ ⨀ τ~1 distance r × × × × × × ×

12. Internal dissipation The prompt GRB Afterglow Prompt GRB Central engine External interactions ~1017-1018cm ~106cm ~1011-1017cm

13. Different radiative mechanisms depending on the location of the energy dissipation Case 2: Photospheric dissipation Case 1: Thomson thin dissipation Where is the prompt emission produced? • in principle anywhere between the Thomson photosphere rph (or slightly below) and the deceleration radius rd • Typically rph~1011cm and rd ~1017cm; in this range of radii: • density ~12 orders of magnitude • optical depth ~6 orders of magnitude

14. Dissipation in the Thomson thin regime • Shocks accelerate particles and amplify magnetic fields • Big variety of spectra depending on the various parameters: • єdiss -fraction of dissipated energy • єB, єe -fraction that goes to B-fields, fast electrons • Fraction ζ of accelerated electrons • Electron power-law index p • Distance of collision • Dominant processes: Synchrotron; synchrotron-self-Compton • Similar for magnetic reconnection at optically thin conditions! Bosnjak, Daigne & Dubus 2008 E*f(E) E*f(E)

15. Photospheric emission • In the fireballthe photospheric luminosity ise.g. Mészáros & Rees 2000 • Spectrum quasi thermalGoodman 1986 (but not exactly black-body Beloborodov 2011) • Energy dissipation (shocks, collisional heating) at τ≥ 1 distorts the spectraMészáros & Rees 2005; Pe’er et al. 2006 • In the reconnection modelDG 2006; DG & Spruit 2007

16. Photospheric emission from the reconnection model • If fraction fe ~ 1 of the energy goes into heating the electrons then • heating-cooling balance gives the electron temperature everywhere in the flow • Resulting emission spectrum with DG 2006; DG & Spruit 2007; DG 2008 • Peak in the sub-MeV range • Flat high-energy emission • observed low-energy slope • Rather high efficiency Lph ~ 0.03…0.5L, for100 < η < 1500 ~ ~

17. Dissipative photospheres: reconnection model synchrotron emission Fermi Swift η=1000 τ<<1 η=590 Robotic telescopes typically observed η=460 E (MeV) η=350 τ~1 η=250 Compton scattering DG 2006; DG & Spruit 2007; DG 2008 more models: Pe’er et al. 2006; Ioka 2010; Lazzati & Begelman 2010; Beloborodov 2010; Ryde et al. 2011

18. From the central engine to radiation Millisecond magnetar Spectrum typical GRB η η Metzger, Giannios, Thompson, Bucciantini & Quataert 2011

19. More dissipative photospheres collisional heating; Beloborodov 2010; Vurm et al. f(E) E*f(E) Pe’er et al. 2006 f(E) weak shocks; Lazzati & Begelman 2010

20. Recent Developments: GeV emission LAT emission: peaking with (late) MeV but lasts longer! counts time Ghiselini et al. 2009 GRB 080916C; Abdo et al. 2009 Physical origin of GeV emission is (in part?) different from the MeV

21. What to make of Fermi observations? • LAT ‘sees’ two components (physically separated) • prompt • slow declining • Need to disentangle them before constraining for the prompt emission cite! • cannot assume a single emission cite for MeV and GeV (e.g. Zhang & Pe’er 2009) GBM LAT L time

22. Correlations: what do we see? Yonetoku et al. 2004 Involve both time integrated and instantaneous quantities (e.g., ) also Borgonovo & Ryde 2001; Liang et al. 2004; Ghirlanda et al. 2004; Liang & Zhang 2006; Firmani et al. 2006; Collazzi & Schaefer 2008… Firmani et al. 2009 Amati 2010

23. Correlations: what can we learn? Tendency for brighter bursts to be cleaner? Transparency of fireball emerging from a collapsar? Metzger et al. 2011; see also DG & Spruit 2007 Ave Peak Energy Epeak Morsony et al. 2011; Lazzati et al. 2011; see also Thompson et al. 2007 Interpretations within photospheric models for Epeak Peak Isotropic Jet Luminosity (erg s-1)

24. Summary • The prompt emission most likely comes from internal dissipation of energy in the fast flow • Internal shocks or Magnetic dissipation or … • Dissipation may take place in Thomson thin or thick conditions • Thin case: particle acceleration uncertainties єe, ζe, p, єB • The photospheric interpretation for MeVs is robust • Magnetic reconnection provides a promising process to power a dissipative photosphere